tests, imsm: Calculate expected array_size in proper way

tests, imsm: Calculate expected array_size in proper way

Tests should calucalte expected array_size accordingly to raid level. Also
tests should take care about runding to neares MB introduced from b53bfba6
"imsm: use rounded size for metadata initialization".

Expect proper size in tests. Simplify 09imsm-overlap test by creating array
with size which has not been rounded. Main purpose of this test is checking
something else.

Signed-off-by: Michal Zylowski <michal.zylowski@intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>